1. Field of the Invention
The present invention relates to bipolar semiconductor devices, and in particular to said devices designed for power utilization.
More particularly, the present invention regards the manufacture of a semiconductor device having a matrix array of cells operating as emitter regions. With this structure a high number of small components are constructed, for example elementary transistors, which are parallel operating, and whose electric functions contribute proportionally to form the current and voltage characteristics of the resulting device.
As the single cells are of regular disposition and size, a perfect balancing of the structure is obtained, from which a certain number of very important advantages derive.
The current loads are equally divided in an homogenous manner in comparison to the usual structures of planar transistors having interdigitated base and emitter. Consequently, the device is stronger and has a higher switching speed.
2. Description of the Prior Art
A semiconductor device produced with a matrix array of cells working as emitters is described for example in the European Patent Application 186140.
In this publication a power switch is shown, which can be transistor or a GTO thyristor, in which a plurality of emitter regions are distributed on the base according to a matrix configuration and parallel connected by means of conductors formed of a layer of metal.
The production of a similar cellular structure, although simple in concept, is in fact limited by technological difficulties, essentially connected to the quality and quantity of the conducting and insulating materials to be deposited.
A particular difficulty in the manufacture, which causes an important limitation in terms of performance and robustness, is the necessity of realizing the configuration with a suitable separation of the base and emitter metallization.
In order to solve this problem, according to the present invention, the two metallizations are superposed vertically, and between the two layers of metal an intermediate dielectric layer is interposed. It is thus possible to realize a structure in which the emitter is formed by a set of single cells, spread out on the base area.
A device denominated SIRET (Siemens Ring Emitter Transistor) is described in a communication published on the occasion of its presentation at IEDM 85.
This device, which consists in a high voltage bipolar power switch transistor, shows a structure with a first metallization configuration in contact with the base, in the form of a mesh around emitter cells. Above said metallization is an insulating layer of silicon nitride, above which there is a second layer of emitter metal. The emitters are placed according to a matrix configuration, forming cells.
In this structure the cells are open and empty, there being no physical contact between the two layers of metal in corrispondence with said cells. The realization of this structure presents particular construction problems for the complexity of the working phases, due to the materials employed.